The following account of the prior art relates to one of the areas of application of the present invention, fibre lasers comprising fibre Bragg grating(s) such as optical fibre distributed Bragg reflector (DBR) or distributed feedback (DFB) lasers.
Bragg-grating based optical fibre lasers such as DBR or DFB lasers are optical fibre lasers which e.g. are produced by UV-imprinting a Bragg grating into a photo sensitive optical fibre which has been doped with an optically active agent, e.g. a rare earth ion such as erbium, ytterbium, and others (cf. e.g. WO-98/36300). Typical dimensions of Bragg-grating based optical fibre lasers along the fibre axis are a few millimeters to a few centimeters.
Bragg-grating based optical fibre lasers may combine attractive features such as stable single mode operation, narrow linewidth and long coherence length, tuning capability, wavelength selection, mechanical robustness, small size, low power consumption, and insensitivity to electromagnetic interference (EMI).
For most applications, including e.g. wavelength tuning, a Bragg-grating based optical fibre laser is packaged under tension in its longitudinal direction, typically affixed to a length-controlling, preferably relatively stiff, substrate. The mechanical properties of the substrate control the length of the optical fibre laser (and may stabilise the optical fibre medium) and consequently control the centre wavelength of the optical fibre laser. The mechanical properties of the substrate have a major influence on the environmental sensitivity of the laser.
For a number of applications a further improvement in coherence length or equivalently a low frequency and/or phase noise is desirable.
The coherence length and the frequency and phase noise properties of Bragg-grating based fibre lasers are influenced negatively by environmental effects such as temperature and acoustic vibrations.
Temperature variations cause variations in the refractive index via the thermo-optic effect. In silica fibres with a thermo-optic coefficient of approximately 10−5° C.−1, Bragg-grating based fibre lasers exhibit a temperature sensitivity of the centre wavelength of about 0.01 nm/° C. At 1550 nm this corresponds to a frequency variation of more than 1 GHz/° C.
Although long term temperature drift can be compensated by specialised packaging techniques involving structures with negative thermal expansion coefficients as e.g. described in WO-99/27400, small and rapid temperature fluctuations cause jitter in the centre frequency corresponding to an increase in the linewidth or a reduction in the coherence length.
Another important contribution to jitter and linewidth increase comes from acoustic perturbations (or mechanical vibrations in general). The linewidth and coherence length of lasers, including single frequency rare-earth doped fibre lasers are ultimately determined by optical spontaneous emission noise, corresponding to the Shawlow-Townes limit. For rare-earth doped fibre lasers this lies in the Hz region. In practical implementations, however, environmental effects such as those mentioned above will affect the cavity stability and lead to linewidths well above the Shawlow-Townes limit. E.g. the thermo-optic effect will lead to frequency shifts of the order of 10−5° C.−1·ν·ΔT [Hz] where ν is the optical frequency (in Hz) and ΔT is the temperature change (in ° C.). As an example, if the frequency stability is required to be better than 1 MHz at 1550 nm then the temperature fluctuations must be lower than 10−3° C. (1 mK).
In order to stabilise the laser frequency and increase its coherence length it is thus necessary to protect it from environmental influences.
Reduction in frequency/phase noise can be obtained by mounting the fibre laser in the neutral axis of a substrate. The neutral axis of a substrate is the axis that is subjected to no strain under bending deformations. In this way, if the substrate design is correct and the fibre laser is mounted in the neutral axis, the effect of vibrational excitations of the substrate on the fibre laser will be significantly reduced (cf. e.g. Hansen, L. V., “Constant Frequency Condition of Fibre Lasers in Strain”, In proceedings, NSCM 15, 15th Nordic Seminar on Computational Mechanics, Eds.: Lund, E.; Olhoff, N.; Stegmann, J., pp. 185-188, October 2002, Aalborg, Denmark, referred to as [LVH-2002] in the following).
Substrates for fibre laser packaging are typically elongated structures that can be considered as (mechanical) beams. The theory used today for simple modelling of beams was mainly developed by Jacob Bernoulli and Euler in the 18th century. Deformations of beams can be split into three parts:                bending deformations,        axial deformations, and        torsional deformations.        
For long beams with a large aspect ratio (i.e. the ratio between length and cross sectional dimensions), deformations due to bending are at least an order of magnitude larger than the axial and torsional deformations. Thus, in addressing methods to suppress acoustic coupling, only bending needs to be considered to first order. In pure bending, one side of the substrate will be in compression, while the other side will be in tension. A neutral axis where the deformation is zero exists between these extremes. If the fibre laser is placed on this neutral axis, bending of the package has no effect: no strain is applied to the fibre laser, hence the frequency is left unchanged, and noise from external vibrations is reduced. The exact location within the substrate of this neutral axis depends on the cross sectional geometry and is determined from Bernoulli-Euler beam theory (cf. e.g. the section “Location of Neutral Axis” on page 311-312 in J. M. Gere and S. P. Timoshenko, “Mechanics of Materials”, Fourth SI Edition, Stanley Thornes (Publishers) Ltd., 1999, the book being referred to elsewhere in the application as [Timoshenko]). This theory places the neutral axis in the position where the first moment of the area of the cross section, S, is zero:S=∫Ay·dA=0
Existing substrates/packages have therefore been developed to reduce the sensitivity to both temperature variations and acoustic vibrations. The present application deals with package designs reducing the effect of acoustic vibrations. Variations in temperature are usually slow and therefore controllable by a heat source/sink element.
Typically, the fibre is mounted on the package under tension but only fixed (e.g. with glue) at each end of the laser (cf. e.g. WO-99/27400). The centre part of the fibre laser can therefore loose contact with (in the following termed ‘escape’) the surface of the package, due to the fibre pre-strain. The effect is illustrated in FIG. 2.b. This effect is undesirable in the case where the laser is placed along a neutral axis of the package. In this case escaping will result in a shortening of the laser cavity length and/or the grating period and consequently in a shift of the lasing frequency.
It can, however, be problematic to fix an optical fibre comprising a fibre Bragg grating to the neutral axis of a package without changing the properties of the fibre. Fixing the fibre with glue along the length of the fibre may damage the fine Bragg-gratings in the core of the fibre, because of non-uniform hardening of the glue. The glue hardening process produces a strain-field in the fibre. The non-uniform strain field destroys the periodicity of the Bragg-grating(s), and the article (e.g. a fibre laser) of which it forms part no longer functions as intended.
It is therefore of interest to provide a scheme for mounting a fibre (e.g. comprising a fibre laser) in a package that overcomes the above problems.
As indicated in the discussion above, out of the three deformations of beams (bending deformations, axial deformations, and torsional deformations), only bending needs to be considered to first order. However, to further improve phase noise in fibre lasers to match required standards in some demanding sensor applications (e.g. use in noisy environments such as aircraft and ships), it becomes necessary to also include/reduce effects of torsional deformations.
In some DFB fiber laser applications a tuning of the frequency/wavelength of the DFB fiber lasers is needed. Existing packages/substrates either use thermal expansion of the package or a package design where the whole package is made of piezo-ceramic material. Due to the relatively high thermal expansion coefficient of Aluminum (about 23*10−6° C.−1) a large wavelength tuning can be obtained by heating or cooling the Aluminum packages. However, the lasing frequency/wavelength can only be slowly modulated by thermal expansion. In some applications where fast modulation is desirable, piezo-ceramic material can be used. When charged electrically, a very fast modulation (in the kHz region) can be obtained by the piezo-ceramic material. However, compared to temperature tuning, only a small frequency/wavelength change is obtainable. When the whole package is made of piezo-ceramic material only a small thermal tuning can be obtained due to its relative low thermal expansion coefficient (about 1-5*10−6° C.−1). There is thus a need for a package with improved tuning options.
U.S. Pat. No. 4,795,226 describes a length of a passive optical fibre with a diffraction grating accommodated in a curved groove of a support block structure. The purpose of the mounting is to provide an appropriate means of polishing away a controllable part of the fibre in a longitudinal direction. The use of the polished fibre is for a device for sensing variable deformations in the fibre (i.e. the aim is to make the grating as sensitive as possible to vibrations from the environment).
U.S. Pat. No. 6,240,220 describes a tunable optical fibre package comprising a curved support member for accommodating a passive optical fiber in channel and piezo electric segments for varying the tension within a fibre Bragg grating to a controlled strain thereby controlling the characteristic wavelength of the grating. The purpose of the package is to vary the wavelength response of the grating according to need. A relatively large tuning is aimed at implying a relatively small radius of curvature of the support member.
US-2002/0131709 describes a device comprising a tunable fibre Bragg grating. A passive optical fibre comprising a fibre Bragg grating is mounted on a substrate that is adapted to be bent by the application of a force perpendicular to the length of the fibre, thereby increasing or decreasing the radius of curvature of the fibre comprising the grating and thus tuning the grating wavelength. The aim of the invention is to make the device as sensitive to the change of radius of curvature as possible to increase the tuning range of the grating.
US-2002/0181908 describes a package for a fibre laser wherein the fibre laser is placed in a tube of a suitably stiff material that has been preshaped to fit into a a suitable size box. The ends are sealed with a suitable glue and the laser in the tube is positioned in the box surrounded by a curable viscous substance.